U.S. Pat. No. 4,078,787 shows a commercially known electrophotographic high volume copier in which a photoconductive belt is entrained around a series of rollers. Toner images are formed on the belt electrophotographically and are transferred to a receiving sheet which is brought into contact with and carried by the belt for a portion of its path. The belt is entrained about a one-inch roller just after the transfer station. The receiving sheet has a tendency not to follow the photoconductive belt as it goes around the small roller and is picked up by a transport belt, one edge of which is positioned just above the small roller. The transport belt holds the non-image bearing side of the receiving sheet and transports it away from the photoconductive belt to a fixing device, for example, a roller fuser.
U.S. Pat. No. 4,714,939, Ahern et al, issued Dec. 22, 1987, also shows a high volume copier. However, in order to do duplex copying with a straight receiving sheet path and less handling, an intermediate transfer roller or belt is positioned in transfer relation with a photoconductive belt. A first toner image is transferred to the intermediate member and the receiving sheet is fed between the photoconductive belt and the intermediate member. The first image is transferred to the top side of the receiving sheet and a second toner image is transferred to the bottom side of the receiving sheet directly from the photoconductive belt. With this approach, duplex images can be formed on a receiving sheet with the receiving sheet passing through a straight paper path. Because toner images are electrostatically transferred in opposite directions to opposite sides of the same sheet, the intermediate roller or belt is positioned to separate from contact with the photoconductive belt before the second toner image is transferred to the receiving sheet from the photoconductive belt. This reduces the tendency of the second transfer to affect the toner image already on the receiving sheet. For a variation of this duplexing approach, see also U.S. Pat. No. 4,688,925, Randall, issued Aug. 25, 1987.
A number of references describe a process for making two (or more) color images by creating an electrostatic image and toning the electrostatic image in the presence of a previously created toner image of a different color. U.S. patent application Ser. No. 07/341,452 to Ahern, filed Apr. 21, 1989, and entitled "Color Duplex Reproduction Method and Apparatus", discloses using that process with an intermediate belt or roller to do multicolor duplex toner images using a straight paper path.
U.S. Pat. No. 4,194,829, W. A. Cavagnaro, issued Mar. 25, 1980, is representative of a number of patents which show making duplex copies by transferring a first toner image to one side of a receiving sheet, turning the sheet over without disturbing the first image, transferring a second image to the opposite side and transporting the sheet to a fuser without disturbing either image. Both images are fused simultaneously. Transporting the sheet to the fuser without disturbing the loose toner images is a challenge in this approach which has been solved commercially for most types of receiving sheets using quite sophisticated transporting devices. In all of the above references suggesting use of intermediates to do duplex, a receiving sheet must also be transported to the fuser with a transport device that does not disturb unfixed fuser images on both sides of the sheet.